Heretofore, the following procedure has been employed for producing an attractive three-dimensional pattern on a concrete surface. First, a rigid polyurethane mat (a pattern transfer mat) 12, such as the one illustrated in FIG. 5, is provided. This transfer mat 12 has an effective transfer surface 13 formed with an aesthetic three-dimensional pattern and a flat back surface 14. At the job site or in the field, a parting agent is applied to said transfer surface 13 and the mat 12 so treated is positioned at a given spacing from a casting form 16. Then, the side openings (not shown) and bottom opening between the form 16 and the mat 12 are closed with boards 17 and a concrete material 15 is poured from the top opening. The concrete material 15 is then allowed to cure and harden to give a concrete board or structure 18. As shown in FIG. 7, the mat 12 is then detached from the concrete 18. The surface of the concrete 18 thus obtained has the three-dimensional pattern of said mat 12 copied on its surface. Finally this sculptured concrete surface is finished, for example, by spraying with a suitable coating composition, to give a natural-looking surface.
However, since the coating material used in the above process is usually an organic composition and the organic coating film tends to discolor or fade or undergo degradation with time, the surface of the concrete cannot be as durable as the surface of, for example, a masonry of inorganic material (such as natural rock). Moreover, it is necessary to recoat the surface at certain intervals and the cost of maintenance including this "refreshing" work is substantial.
To overcome these disadvantages it has been proposed to employ a transfer mat carrying natural pebbles or glass fragments secured with a water-soluble adhesive, casting concrete against this transfer mat, removing the mat from the cured concrete to thereby leave said natural pebbles or the like embedded in the concrete and finally washing out the water-soluble adhesive (Japanese Patent Kokai Publication No. 56-169189/1981). However, this technology has the drawback that in casting concrete against the effective surface of the transfer mat, delicate air cells remain entrapped on the mat surface and produce pits or small cavities in the surface layer of the concrete, thus detracting from the finished appearance of the cast concrete. This drawback is particularly prominent when the transfer mat has an undulating or profiled surface.